This invention relates generally to the fabrication of glass preforms to be used in the molding of glass optics, and more particularly, to methods and apparatus for making cylindrical glass preforms having optical quality convex ends from glass ball preforms.
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Engleet al., U.S. Pat. No. 4,139,677 to Blair et al., and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of mold inserts used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the molds include glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity with the mold being formed out of one of the above mentioned materials. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to a viscosity ranging from 107-109 poise for the particular type of glass from which the preform has been made. Pressure is then applied to force the preform to conform to the shape of the mold cavity. The mold and preform are then allowed to cool below the glass transition temperature of the glass. The pressure on the mold is then relieved and the temperature is lowered further so that the finished molded lens can be removed from the mold.
Because the molding of glass optical elements is done by compression rather than injection (as is utilized in plastic molding) a precursor metered amount of glass, generally referred to as a preform, is required. There are two fundamental shapes of preforms required which generally parallel the fundamental finished lens shapes. For negative lenses (negative refers to a concave optical surface) plano-plano preforms usually will be sufficient. These can be fabricated in high volume relatively inexpensively by grinding and polishing. For positive lenses (positive refers to a convex optical surface) a ball (sphere) or ball-like lump of glass is needed.
Traditionally in molding glass lenses, the outside diameter of a glass-molded lens is not constrained. A glass preform is placed on the lower mold. The mold is heated until the glass is soft, and the two molds are pressed together. Any excess glass material is allowed to freely flow beyond the desired diameter of the lens. The mold is then cooled and the molds are separated to produce a finished molded lens. The variation in the outside diameter from lens to lens is proportional to the variation in the volume of the preforms that are used to mold the lenses. To produce finished molded lenses with very tight tolerances on the outside diameter requires very tight control of the preform volume.
When molding precision glass lenses, preforms are often used which approximate the size and shape of the finished product. Since precision lens molding is typically performed while the glass is yet somewhat rigid, any imperfections found on the preforms become magnified during molding as the surface area of the glass is increased. These preforms can become quite expensive to manufacture since the surfaces that are eventually used for imaging must be polished to optical quality in order to produce superior finished products. In the case of positive lens surfaces, which are characterized by a convex lens surface and a concave mold surface, hemispherical ended preforms are often used to approximate the shape of the convex spheric or aspheric lens surface and to reduce the amount of glass flow during the molding cycle. Glass flow is intentionally minimized in order to reduce cycle time, reduce internal and surface stress in the material, and limit the creation of cosmetic surface flaws in the final part.
In a production level glass lens molding process, it is advantageous to use preforms that are symmetric in shape, since part orientation will then be eliminated from the handling process when loading the mold assembly. If notable differences are found between the optic forming surfaces of the preform, the preform is considered to possess a low degree of symmetry and the final lens may not be producible within acceptable tolerance specifications. Therefore, in many cases, the production of high quality preform surfaces is the primary contributor to final lens cost.
There are many applications where it is desirable to mold a glass optical lens such that the outside diameter of the lens is concentric with the optical axis of the lens. The deviation of the distance from the outer radial surface of a lens to the optical axis of the lens is called runout. If the lens is later assembled in a barrel, it is desirable to have the runout minimized such that the optical axis of the lens is colinear with the axis of the barrel. Traditionally, lenses used for imaging purposes (cameras, telescopes, etc.) were designed such that their diameter was much greater than their center thickness. High aspect ratio parts such as these were easy to handle and to mount in a variety of subassemblies.
However, the emergence of fiber optic telecommunications, as well as the micro lens market, has created a need for lenses with very low aspect ratios. The ability to control the outside diameter and runout is especially useful when the shape of the lens is cylindrical (with an aspect ratio  less than 1) or, when the diameter of the lens is less than about 2 mm. It is particularly difficult to precisely grind the outside diameter of a lens with a diameter of less than about 2 mm. These small cylindrically shaped lenses are particularly useful as collimator lenses of the type used in fiber optic transmission. Lenses of this geometry cannot easily be made from spherical preforms, and so another low cost method of preform manufacture is required for these lenses to be made economically. The shape that most closely resembles the final lens is that of a right, circular cylinder with spherical ends which, in turn, minimizes glass flow.
It is therefore an object of the present invention to provide a method and an apparatus for forming cylindrical glass preforms with convex optical quality ends or end surfaces for later use in molding glass optical elements therefrom.
It is a further object of the present invention to provide a method and an apparatus for forming cylindrical glass preforms from generally spherical or ball glass preforms.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by rolling a generally spherical glass or ball preform of a suitable volume that has been heated to yield a viscosity of between 104 and 108 poise between substantially parallel platens. The generally spherical glass preforms can be economically produced with very accurate volume control by traditional grinding and polishing methods, or by any other of the numerous and well established methods of glass ball formation. See, for example, U.S. Pat. No. 5,709,723 to Gearing et al. Therefore, the final preforms produced from these balls will also have precise glass volume control. In the practice of the present invention, a generally spherical or ball preform is placed in a heated venturi. A flow of nitrogen gas suspends the glass preform so that it does not contact the sides of the venturi, since the highly polished optical quality surface of the preform may be negatively affected by contact with the surfaces of the venturi. Once the glass ball preform is heated to achieve the required viscosity (104-108 poise) for rolling, the nitrogen gas flow is interrupted, allowing the glass ball preform to drop onto a lower platen that is pre-heated to a level near that of the glass transition temperature. The temperature of the platen should be maintained to a level within approximately 30xc2x0 C. above the glass transition temperature (Tg). This value will vary depending on the viscosity characteristics of the glass. A xe2x80x9cshortxe2x80x9d glass will require closer monitoring, while a xe2x80x9clongxe2x80x9d glass will be easier to control. Once the glass ball preform is on the lower platen of the rolling chamber, the lower platen (which may or may not be moving horizontally when the glass ball preform is dropped thereon) is indexed horizontally to an initial rolling horizontal position. Once in the initial rolling position, the lower platen is indexed to an initial rolling vertical position (setting a predetermined gap between the upper platen and the lower platen) where the preform is engaged by the preferably stationary upper platen. From this position, the lower platen is moved horizontally which causes the glass ball preform to roll relative to the upper platen. As the lower platen is moved horizontally it is also gradually raised to thereby narrow the gap between the upper and lower platen. In this manner, as the glass preform is rolled between the upper platen and the lower platen, it is gradually subjected to a compressing force. The rolling action, in combination with the compressing force, causes the glass ball preform to be reshaped into a cylindrical body with convex ends. Once a predetermined final diameter for the cylindrical preform has been achieved, the vertical movement of the lower platen is halted. In other words, the upward movement of the lower platen stops when a predetermined final gap between the upper platen and the lower platen is reached. It is this predetermined final gap that determines the final diameter for the cylindrical preform. Once the final cylindrical diameter of the cylindrical preform is achieved, preferably, as the cylindrical glass preform traverses the lower platen, it is exposed to a decreasing temperature gradient that serves to raise the viscosity of the preform to a stage above its Tg value. The result is a cylindrical glass preform having convex ends. The surfaces of the convex ends will not have been contacted by the platens and, as such, the surfaces of the convex ends will retain their optical quality. Such convex ends should be generally spherical, but may be aspherical, as a result of the actual shape of the glass ball preform used, or as a result of some minor deformation during the rolling process. However, in either case, the convex ends will be convex and substantially symmetrical. The cylindrical preform is then removed from the lower platen. Preferably, removal is accomplished by continuing to move the platen until the finished cylindrical preform with convex ends or end surfaces is automatically rolled off the end of the lower platen and into a receiving container. The platens then return to their home positions in preparation to accept the next heated glass ball preform.
The platen surfaces are fabricated and/or coated with a ceramic or refractory material that resists glass wetting and is amenable to working hot glass (such as boron nitride, graphite, vitreous carbon, or any noble metal such as platinum or gold). Based on the temperatures and the tool materials, an inert gas purge may be required to prevent tool surface degradation.
In another version, the glass ball preforms may be hot gobbed directly onto the lower platen, thereby eliminating the need for a heated venturi.
Those skilled in the art will understand that although the glass ball preforms used in the present invention are described as being generally spherical, such glass ball preforms will rarely, if ever be truly spherical, particularly if formed by hot gobbing. Similarly, the cylindrical glass preforms formed with the method of the present invention, although described herein as having generally spherical ends or end surfaces, will rarely, if ever have truly spherical ends or end surfaces. Thus, the term xe2x80x9cgenerally sphericalxe2x80x9d as used herein is intended to mean ball shaped or ball-like.